U.S. Pat. No. 3,390,997 discloses a light-sensitive admixture of an alkylthio, benzylthio, 2-phenylhydrazino or alkoxycarbonyl derivative of a triphenylmethane compound (a "leuco dye") and a selected non-volatile nitrogen-containing compound which functions as a photooxidant useful as an imaging system. The patent does not disclose the use of diazonium salts or materials which contain a pentavalent nitrogen atom. The light sensitive compounds of the present invention differ significantly from those of the patent which contain only trivalent nitrogen atoms. Furthermore, the present invention requires elevated temperatures (180.degree. F. [82.degree. C.] to 380.degree. F. [193.degree. C.]) for image development, whereas the patent disclosure is of a room temperature developing system.
U.S. Pat. Nos. 3,445,233 and 3,215,529, and Abstracts of Japanese Pat. No. 78-102,038 and Japanese Document No. 51-51942 disclose imaging systems containing diazonium salts but in no case do they react directly with leuco dyes. Great Britain patent specification Nos. 1,041,463 and 1,170,458 disclose diazonium salts interacting with acid-base indicators.
An imaging system comprising a leuco dye, a diazonium salt, and nitrate ion in a binder is described in assignee's now allowed copending continuation-in-part patent application, U.S. Ser. No. 200,323, filed Oct. 24, 1980, the parent of which is U.S. Ser. No. 101,196 filed Dec. 7, 1979, now abandoned. This four-part imaging system does not suggest the efficacy of the three-part imaging system of the present invention, and the chemical mechanisms in these diverse systems is believed to be different.